Electrical transformer with zero sequence voltage indicator

ABSTRACT

A three-phase electrical transformer of the core-form type including wye-wye connected primary and secondary windings having grounded neutrals. The magnetic core has three winding legs for the primary and secondary windings, and at least one additional leg. An auxiliary winding is disposed on the additional leg and connected to indicating means which provides a signal when the voltage induced into the auxiliary winding by zero sequence flux flowing through the additional leg reaches a predetermined magnitude.

United States Patent I 1 I I 1 1 1 Inventors David R. Smith McKeesport.Pa.; Alvin Y. Broverman, Crystal Springs, Miss. Appl. No. 76,5 FiledSept. 29, I970 Patented Nov. 30, 197i Assignee Westinghouse ElectricCorporation Pittsburgh, Pa.

ELECTRICAL TRANSFORMER WITH ZERO SEQUENCE VOLTAGE INDICATOR 8 Claims, 3Drawing Figs.

U.S. Cl .1 324/86,

317/18 D, 324/108, 340/255 Int. Cl G0lr 25/00 Field of Search 336/5, 10,

12,214,215; 324/86, 108;3l7/l8 R, 18D

TO SOURCE OF ALTERNATING POTENTIAL w TO LOAD CIRCUIT [56] ReferencesCited UNITED STATES PATENTS l,535,588 4/1925 Evans 324/86 X 2,068,5751/1937 Stank 324/86 X 2,922,972 1/1960 Gordy 336/5 X 3,504,318 3/1970Wilburn et al.. 336/215 X 3,509,507 4/1970 Specht 336/215 X PrimaryExaminer-Thomas .l Kozma Attorneys-A. T. Stratton, F. E. Brosder and D.R. Lackey ABSTRACT: A three-phase electrical transformer of the coreformtype including wye-wye connected primary and secondary windings havinggrounded neutrals. The magnetic core has three winding legs for theprimary and secondary windings, and at least one additional leg. Anauxiliary winding is disposed on the additional leg and connected toindicating means which provides a signal when the voltage induced intothe auxiliary winding by zero sequence flux flowing through theadditional leg reaches a predetermined magnitude.

ZERO SEQUENCE VOLTAGE lNDlCATOR PATENTEDNUV30I97| 3.624.499

TO SOURCE OF ALTERNATING POTENTIAL 48 so s2 ZERO SEQUENCE VOLTAGEINDICATOR 84 FIG. I.

w TO LOAD CIRCUIT as 62 m ZERO SEQUENCE a4 VOLTAGE INDICATOR L LQ L 2 (DI214 |2 6 128 (1) l -J 150 i: :j |52 |3e '50 I32 I34 FIG. 3

FIG. 2

WITNESSES: mvsm'ons David R. Smith and Alvin Y. Brovermon.

ATTORNEY ELECTRICAL TRANSFORMER WITH ZERO SEQUENCE VOLTAGE INDICATORBACKGROUND OF THE INVENTION of the wye connection short circuits andminimizes the probability of ferroresonance.

The grounded wye-wye connection of the transformer, however, provides apath for zero sequence currents to flow during unbalanced loadconditions, open phase conditions, or during line-to-ground faults.Since the zero sequence fluxes flowing cause serious heating of thesecomponents.

Disposing conventional tertiary windings on each winding leg andconnecting themin a delta configuration, is not a satisfactory oreconomical solution in the usual pad-mount installation. The zerosequence impedance of this delta connected tertiary winding, withrespect to the other windings, will usually be so low that the magnitudeof the zero sequence current flowing upon the occurrence of a faultlocated between the primary windings of the transformer and theelectrical generator or power source, may cause one or more of thetransformer primary fuses to blow, instead of clearing the fault solelywith the protective apparatus designed for this purpose at the source.Designing the tertiary winding to have the proper impedance would becostly, and therefore an undesirable solution to the problem.

For these reasons, it is conventional to utilize either a fiveathree-phase core-form 824,376, now U.S. Pat. No. 3,504,318, filed Dec.30, i968 and May 7, 1969, respectively, a four-legged magnetic core maybe used to advantage over a five-legged magnetic core. With either typeof core, the primary and secondary windings are connected in wye andgrounded. With a five-legged core, the three intermediate legs are thewinding legs, and the two outer legs provide return paths for any zerosequence flux which flows in the winding legs of the core. With afour-legged magnetic core, any three legs may be used for winding legsand the remaining leg provides a return path for any zero sequence flux.The reluctance of the additional leg, or legs, to

The zero the magnitude of the zero sequence current and prevent thetransformer fuses from blowing upon a line-to-ground fault locatedbetween the power source and the transformer primary windmg.

The presence of zero sequence flux in the additional leg, or legs, ofthe magnetic core signifies an unbalanced load, an open phase. or afault condition, and when the zero sequence provide signals which may beused to indicate open phases and fault conditions, but the cost of thisapproach outweighs the benefits obtained.

SUMMARY OF THE INVENTION Briefly, the present invention is a three-phasetransformer of the core-form type having grounded wye-wye connected andone or two additional legs. In the embodiment of the inback to thecentral station, or both, as desired.

In the embodiment of the invention having two additional legs, i.e., afive-legged core, an auxiliary winding is disposed on each additionalleg and serially connected such that zero sequence fluxes in these twolegs provide additive voltages in the serially connected windings.Normal flux in these legs will cancel in the serially connectedwindings, thus providing a voltage indicative of the magnitude of thezero sequence flux flowing in the winding legs. The two seriallyconnected BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and usesof the invention will become more apparent when considered in view ofthe following detailed description of exemplary embodiments thereof,taken with the accompanying drawings, in which:

FIG. 1 is an elevational view of a three-phase transformer of thecore-form type, constructed according to a first embodiment of theinvention;

FIG. 2 is a schematic diagram of zero sequence voltage indicating meanswhich may be used to provide a local and remote indication of zerosequence flux; and

FIG. 3 is an elevational view of a three-phase transfonner of thecore-form type constructed according to another embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, there is shown a three-phase transformer 10constructed according to the teachings of the invention. Transformer 10includes a magnetic core structure 12, shown in an elevational andwinding phases l4,

and secondary windings 20 and 22, respectively, phase 16 includesprimary and secondary windings 24 and 26, respectively, and phase 18includes primary and secondary windings 28 and 30, respectively.Transformer 110 is of the three-phase, core-form type, with its primaryand secondary windings being connected in a grounded wye-wyeconfiguration.

Magnetic core 112 includes first, second and third winding leg members34, 36 and 38, and an additional leg member 40, all disposed in spacedparallel relation, having their ends joined by upper and lower yokemembers 42 and 44, respectively. Leg members 34, 36, 38 and 40 are eachformed of a plurality of stacked metallic, magnetic laminations, such assilicon, cold-rolled electrical steel, having at least one preferreddirection of magnetic orientation. The ends of the laminations whichmake up the first winding leg member 34, and the additional leg member40, may have their ends cut diagonally, for example, at an angle of 45with respect to the longitudinal dimension of the lamination, and theends of the laminations which make up the second and third winding legmembers 34 and 36 may be cut form spear points," with two diagonal cutsat each end.

For purposes of example, the additional or fourth leg member 40 isdisposed outside of the winding leg members, i.e., at one end of themagnetic core, such as adjacent winding leg member 38, as shown in F116.ll, but it may be disposed between any two winding leg members, ifdesired. Unlike leg members 34, 36 and 38, leg member 40 does not havepower windings disposed thereon.

The upper and lower yoke members 42 and 44, respectively, areconstructed of a plurality of layers of metallic, magnetic laminationssimilar to the leg members 34!, 36, 38 and as, with each layer havingone or more laminations. The ends of the outer laminations of both theupper and lower yoke members are cut diagonally to match the diagonalcuts on the laminations of the outer leg members 34 and 40. Each layerof laminations of the upper and lower yoke members 42 and 44 alsocontains an appropriate notch or indentation formed by two adjacentlaminations, or cut in the side of a single lamination, which matchesthe spear points of the two intermediate leg members 36 and 38.

The joints formed in each layer of laminations between the various legand yoke members may be offset from similar joints in succeeding layersin a predetermined stepped-lap pattern, as disclosed in US. Pat. No. 3,1 53,2 l5, which is assigned to the same assignee as the presentapplication, or any other suitable joint arrangement may be used, suchas the butt-lap configuration.

Leg members 34, 36 and 38, and the portions of the upper and lower yokemembers 42 and 44 which join them, are dimensioned the same as theywould be in a conventional three-legged magnetic core of the samerating, with the width W of the leg members being the same. Theadditional leg member 40 is shown as having the same width W as thewinding legs 34, 36 and 38, which is its maximum width. Systemconditions may make it possible to reduce the width of the additionalleg member 40, but since it will facilitate manufacturing to use thesame lamination width for all leg members, for practical purposes itwill usually have the same width W as the winding leg. As shown in FIG.1, the yoke laminations have the same width W as the winding leglaminations. However, yoke widening may be used when necessary toincrease the yoke dimension'between the outer sides of the yoke membersand the ends of the spear points of the intermediate leg members 36 and38.

The remainder of the magnetic core 12 from winding leg 38 to leg 40 isdifferent than the conventional three-legged magnetic core, with thisportion of the upper and lower yoke portions being lengthened in orderto accommodate the fourth leg member 40, and provide a window forreceiving the winding on winding leg member 38.

The primary and secondary windings are connected in a grounded wye-wyeconfiguration, with the primary winding including phase winding sections20, 24 and 28 disposed about winding legs 34, 36 and 38, respectively.Phase winding sections 20, 24 and 28 have one end connected to terminals458,

50 and 52, respectively, which are adapted for connection to athree-phase source of alternating potential, and the other ends of thephase winding sections are connected in common at neutral terminal 54,which is grounded at 56.

The secondary winding includes phase winding sections 22, 26 and 30,which are disposed about winding leg members 34, 36 and 33,respectively. Phase winding sections 22, 26 and 30 have one endconnected to terminals 63, 70 and 72, respectively, which are adaptedfor connection to a three-phase load circuit, and their other ends areconnected in common at neutral terminal 74, which is grounded at 76.

While the primary and secondary phase winding sections on each windingleg are shown axially spaced apart, they will usually be disposed inconcentrically adjacent, spaced relation.

Pad mounted power transformers connected into the electrical system viaunderground shielded cables present ferroresonance problems due to thecapacitance of the shielded cable to ground. The incidence offerroresonance is substantially reduced by connecting the windings in agrounded wyewye configuration, as shown in FIG. 1. This connection ofthe windings, however, provides a path for zero sequence currents toflow during unbalanced load conditions, open phase conditions, andduring fault or short circuit conditions, and thus some provision shouldbe made for the zero sequence flux.

For balanced three-phase loading, the vector sum of the threephase-to-ground voltages and therefore of their fluxes, is equal to zeroat any instant. Thus, during normal loading conditions, the additionalleg member 40 carries no flux. When the vector sum of the threepahse-to-ground voltages at the transformer is not zero, a zero sequencecomponent of the flux flows in each winding leg, and these componentsare all in phase. The zero sequence components of the flux thus flowthrough the fourth or additional leg member 40. The flux (I), flowing inleg 0 is, therefore, directly proportional to the total zero sequenceflux flowing in the magnetic core 12, and is proportional to anunbalance caused by unbalanced loading, line-to-ground faults, or anopen phase.

According to the teaching of the invention, an auxiliary winding isdisposed about the additional leg 40, which winding develops a voltageacross its ends proportional to the magnitude of any zero sequence fluxflowing through leg 40. Winding 80 is connected to input terminals 82and 84 of a zero sequence voltage indicating means 86. The zero sequencevoltage indicating means 86 may be of any desired type, such as a visualor audio alarm at the transformer site, or a signal may be developedwhich is transmitted or relayed back to the central station such as viathe power lines, telephone lines and the like. if the signal istransmitted back to the central station, each transformer may have adifferent frequency signal, on order to identify which transformerlocation is providing the signal.

HO. 2 is a schematic diagram of zero sequence voltage indicating meanswhich may be used for the means 86 shown in FIG. 1. The means utilizedby the electrical utility will depend upon the specific requirements ofthe utility. More specifically, voltage indicating means 86 may includea step-up transformer 90 having a ratio which will provide the necessaryvoltage to operate a relay 92 when the zero sequence flux reaches apredetermined magnitude. Since zero sequence voltages will be developedfor even small unbalances in transformer loading, it would not bedesirable to provide a signal as soon as flux appears in leg 40. Thus,the circuit should be arranged to provide an indication when a certainpercentage of the maximum zero sequence flux is flowing. The maximumzero sequence flux would occur when the voltage of one phase collapsesentirely, so transformer 90 should be selected to operate relay 92 atsome predetermined percentage of this maximum voltage, such as 15 to 20percent thereof. Step-up transformer 90 has a primary winding 94connected to input terminals 82 and 84, and a secondary winding 96connected across the electromagnetic coil 98 of relay 92. An adjustableresistor 100 may be connected to provide an adjustment on the pickuppoint of relay 92.

If it is only desirable to transmit a signal back to the centralstation, a suitable transmitter 102 may be connected in series with anormally open contact 104 of relay 92 and a source 115 of potential,such as the alternating current source illustrated, or a direct currentsource, such as a battery. When relay 92 picks up, contact 104 closesand transmitter 102 provides a signal as long as the condition whichcaused the excessive zero sequence flux persists. If it is desirable toprovide an indication at the transformer site that the transformer hasbeen subjected to a condition which caused excessive zero sequence flux,even though the condition no longer exists, it will be necessary toprovide an additional relay which will seal in the signal means andprovide a continuous signal until the circuit is reset. In thisinstance, a relay 106 is provided having an electromagnetic coil 108 anda normally open contact 110, and means for providing a visual signal,such as an electric light bulb 112. The electromagnetic coil 108 andlight bulb 112 are connected across source 115 via a normally opencontact 114 of relay 92. Contact 110 of relay 106 and a normally closedreset pushbutton 116 are serially connected across contact 114. Thus,when relay 92 is energized and its contact 1141 closes, theelectromagnetic coil 108 of relay 106 will be energized and the lightbulb 112 will be connected to the source 115. Relay 106 seats itselfinto the circuit via its contact 110, and the light bulb 112 will remainenergized even after relay 92 drops out, until the reset pushbutton 116is actuated to open the circuit through electromagnetic coil 108 ofrelay 106.

If a signal proportional to the zero sequence flux is transmitted backto the central station via the transmitter 102, it will not usually berequired to provide a visual signal at the transformer site, but bothtypes of signal means are illustrated in FIG. 2 for purposes of example.When only a visual signal at the transformer site is required, it willnot be of any benefit to the utility when a complete phase is lost, or afault occurs which operates the associated protective apparatus, butthis type of signal indication will provide useful information to theutility for those conditions which are undesirable, but which do notcause a complete outage, such as unbalanced loads or intermittent faultson a transformer, which conditions should be known by the electricalutility in order for them to provide corrective action and preventfuture discontinuity in their service.

FIG. 3 is an elevational view of a transformer 120 constructed accordingto another embodiment of the invention. Transformer 120 is a three-phasetransformer of the coreforrn type having a magnetic core 122 and phasewindings 124, 126 and 128, but instead of a four-legged core, afivelegged magnetic core is used. The phase windings are illustratedonly in dotted outline in FIG. 3, as they each have primary andsecondary windings connected in the grounded wyewye configuration, asshown in FIG. 1.

More specifically, as shown in FIG. 3, magnetic core 122 has threeintermediate winding leg members 130, 132 and 134, which receive thephase winding sections 124, 126 and 128, respectively. Magnetic core 122also has first and second outer leg portions 136 and 138, and upper andlower yoke portions 140 and 142, respectively. The yoke portions 140 and142 join the ends of the spaced parallel leg portions, forming windowsfor receiving the windings disposed on the winding legs. The windinglegs have a dimension W, similar to the winding legs of the magneticcore 12 shown in FIG. 1, but the outer leg members 136 and 138 and upperand lower yoke members 140 and 142 usually have a width dimension equalto about one-half of the winding leg width.

Unlike the four-legged magnetic core shown in FIG. 1, the additionallegs 136 and 138 on the five-legged magnetic core 140 have flux flowingtherein during normal balanced loading of the three phases. However, thezero sequence flux flowing in the two outer legs 34 and 40 during anunbalanced circuit condition may still be detected by disposing firstand second auxiliary coils 150 and 152 on legs 136 and 138,respectively, and connecting the coils or windings such that the voltagepicked up due tothe zero sequence fluxes are additive. The voltagesinduced into the auxi rary windings and 152 due to flux other than thezero sequence flux cancels one another, when the windings are connectedto add voltages due to the zero sequence flux, thus providing a voltageacross the two serially connected coils 150 and 152 which is directlyproportional to the total zero sequence flux in the magnetic core.Auxiliary windings 150 and 152 are connected to the input terminals 82and 84 of the zero sequence voltage indicating means 86, as hereinbeforedescribed relative to FIGS. 1 and 2.

In summary, there has been disclosed new and improved three-phasetransformer arrangements of the core-form type, having grounded wye-wyeconnected primary and secondary windings, and a magnetic core havingeither four or five legs. The arrangements include means for providingan indication of any zero sequence flux flowing in the magnetic core.Since the zero sequence flux is proportional to the vector sum of thethree phase-to-ground voltages at the transformer, providing a signalwhen the zero sequence flux reaches a magnitude considered undesirable,identifies unusual loading and/or fault conditions. Thus, the electricalutility, upon being notified of the condition by the zero sequencevoltage indicating means, may take the necessary corrective actionbefore the unusual condition causes an interruption in service.

We claim as our invention:

1. A three-phase transformer of the core-form type, comprising:

a magnetic core having first, second and third winding leg members.

a first additional leg member,

first and second yoke members disposed to connect the ends of saidfirst, second and third winding leg members, and said first additionalleg member,

first, second and third phase winding assemblies disposed in inductiverelation with said first. second, and third winding leg members,respectively, said first, second and third phase winding assembliesincluding primary and secondary windings connected in a grounded wye-wyeconfiguratron,

a first auxiliary electrical winding disposed about said firstadditional leg member, which produces a voltage responsive to themagnitude of zero sequence flux flowing therethrough,

and indicating means connected to said auxiliary electrical windingwhich provides a signal when the zero sequence flux flowing in saidfirst additional leg member reaches a predetermined magnitude.

2. The three-phase transformer of claim 1 including a second additionalleg member, and a second auxiliary winding disposed about said secondadditional leg member, with the first and second additional legmembersbeing disposed at opposite ends of the magnetic core, the firstand second auxiliary windings being serially connected to add voltagesinduced therein responsive to zero sequence flux, said seriallyconnected first and second auxiliary windings being connected to theindicating means.

3. The three-phase transformer of claim 1 wherein the first additionalleg member is disposed at one end of the magnetic core.

4. The three-phase transformer of claim 1 wherein the first additionalleg member is disposed between two of the winding leg members.

5. The three-phase transformer of claim 1 wherein signal provided by theindicating means persists until reset.

6. Tile three-phase transformer of claim 2 wherein signal provided bythe indicating means persists until reset.

7. The three-phase transformer of claim I wherein the signal provided bythe indicating means is transmitted to a point remote from thetransformer.

8. The three-phase transfonner of claim 2 wherein the signal provided bythe indicating means is transmitted to a point remote from thetransformer.

the

the

1. A three-phase transformer of the core-form type, comprising: amagnetic core having first, second and third winding leg members, afirst additional leg member, first and second yoke members disposed toconnect the ends of said first, second and third winding leg members,and said first additional leg member, first, second and third phasewinding assemblies disposed in inductive relation with said first,second, and third winding leg members, respectively, said first, secondand third phase winding assemblies including primary and secondarywiNdings connected in a grounded wye-wye configuration, a firstauxiliary electrical winding disposed about said first additional legmember, which produces a voltage responsive to the magnitude of zerosequence flux flowing therethrough, and indicating means connected tosaid auxiliary electrical winding which provides a signal when the zerosequence flux flowing in said first additional leg member reaches apredetermined magnitude.
 2. The three-phase transformer of claim 1including a second additional leg member, and a second auxiliary windingdisposed about said second additional leg member, with the first andsecond additional leg members being disposed at opposite ends of themagnetic core, the first and second auxiliary windings being seriallyconnected to add voltages induced therein responsive to zero sequenceflux, said serially connected first and second auxiliary windings beingconnected to the indicating means.
 3. The three-phase transformer ofclaim 1 wherein the first additional leg member is disposed at one endof the magnetic core.
 4. The three-phase transformer of claim 1 whereinthe first additional leg member is disposed between two of the windingleg members.
 5. The three-phase transformer of claim 1 wherein thesignal provided by the indicating means persists until reset.
 6. THethree-phase transformer of claim 2 wherein the signal provided by theindicating means persists until reset.
 7. The three-phase transformer ofclaim 1 wherein the signal provided by the indicating means istransmitted to a point remote from the transformer.
 8. The three-phasetransformer of claim 2 wherein the signal provided by the indicatingmeans is transmitted to a point remote from the transformer.